Organic materials comprising natural polymers, synthetic polymers, fats and oils, lubricant oils and hydraulic oils, etc. are subject to oxidation resulting in lowered utility. Thus, various antioxidants have been devised and added to these organic materials. For example, it has been known that addition of stabilizers such as hindered phenolic compounds, thioalkanoic acid ester compounds, organic phosphorus compounds and aromatic amines solely or in combination can achieve a stabilizing effect.
In particular, synthetic resins such as polyethylene, polypropylene, acrylonitrile-butadiene-styrene resin and poly(vinyl chloride) resin are known to be degraded by the actions of heat, oxygen, light, etc., and result in extremely shortened life time due to color change and lowered mechanical strength. In order to prevent such degradation, there have been developed various kinds of antioxidants. Of those, phosphorus antioxidants have been widely used because of their superiority in preventing color change, and improved heat resistance and weathering resistance of resin given by them. Motivated by the increased demand in recent years for antioxidants having such superior heat resistance as to prevent volatilization at high temperatures, and superior processing stability, which has derived from the use of high temperatures for high speed molding and high temperature molding using alloys with engineering plastics, etc., phosphite and phosphonite compounds have been developed as phosphorous antioxidants and are on the market.
British Patent No. 1372528 discloses 4,4'-biphenylenediphosphonite compounds represented by tetrakis(2,4-di-tert-butylphenyl)-4,4'-biphenylenediphosphonite as a phosphorus antioxidant.
Also, GB A-2215727 discloses a composition containing tetrakis(2,4-di-tert-butylphenyl)-4,4'-biphenylenediphosphonite as a phosphorus antioxidant, and GB A-2227490 discloses a composition containing tetrakis(2,4-di-tert-butyl-6-methylphenyl)-4,4'-biphenylenediph osphonite as a phosphorus antioxidant.
Nonetheless, the widely-employed tetrakis(2,4-di-tert-butylphenyl)-4,4'-biphenylenediphosphonite is not sufficient in terms of hydrolysis resistance and heat stability of the compound itself and resistance to coloration and heat stability of the resin composition containing an active ingredient. In addition, the stabilizing effect by the aforementioned composition is not satisfactory.